1. Field of the Invention
The present invention relates to pumps in general and, more particularly, to pumps used in compressing a fluid.
2. Description of the Prior Art
Multicylinder single acting positive displacement pumps are used to move drilling fluid, water, crude oil, refined petroleum products, etc. Often the output pressure of such a pump will be 1,000 psi or greater. Further the discharge from this type of pump is a variable function of time. It consists of a constant flow, corresponding to the time averaged flow rate, and an oscillating or "alternating current" component. This oscillating flow accounts for the pressure surges observed in the output of these pumps. These pressure pulsations can be of sufficient magnitude to cause severe vibration in the output manifold of the pump. This vibration can be so severe as to cause premature fatigue failures of output piping, resulting in loss of pumped product and/or damage to facilities and personnel.
These pressure pulsations in the pump discharge may be reduced by incorporating a "pulsation dampener" in the discharge manifold. Conventionally such pulsation dampeners consist of a properly sized compliant volume which may or may not be followed by a "choke tube", i.e. a restricted length of pipe leading to the discharge manifold. The purpose of the compliant volume is to provide a surge tank with which the oscillating component of the pump output can interact to produce pressure pulsations that are much smaller than those which result from the interaction of the oscillating output with the unmodified pump discharge manifold.
One common form of a compliant volume pulsation dampener is a nitrogen filled volume separated from the pumped fluid by a flexible bladder or diaphragm. These range in size from one to twenty gallons.
When properly "charged" with nitrogen under pressure, and tuned, these devices can significantly reduce the severity of one or two of the lower frequency pulsations generated by the pump. Unfortunately, if pump speed or discharge pressure is changed, the dampener must be retuned if it is to continue to be effective.
Although the gas-filled dampener is thought of as a frequency insensitive surge tank, measurements indicate that this type of dampener is a highly resonant system, offering significant attenuation over only a very narrow frequency range.
Unfortunately, these gas-charged units are not usually well maintained. Unrepaired, ruptured bladders and improper charge pressures are common. Under these conditions, the dampeners perform poorly.
A second, and far more effective form of compliant volume consists of a large tank completely filled with the pumped fluid. The compliance of such a device depends on the compressibility of the fluid and the size of the enclosed volume. To be effective, these devices must be one or two feet in diameter and ten to twenty feet tall, with capacities up to two hundred gallons. When combined with a choke tube ten to fifteen feet long, these devices make a most effective pulsation dampener.
The totally liquid filled dampener is a large, heavy installation. To withstand discharge pressures in excess of one thousand pounds per square inch, tank wall thicknesses on the order of three to five inches are necessary. Thus, these dampeners may weigh three to four tons each. In addition, the flow restriction through the choke tube will result in pressure losses on the order of thirty to fifty pounds per square inch.
The present invention provides effective dampening of two lower pulsation frequencies generated by a multicylinder single acting pump. These frequencies have been shown to be the primary cause of fatigue failures in discharge manifolds. The present invention provides effective dampening independent of pump speed, and does not require retuning if the pump speed changes.